1. Field of the Invention
The present invention relates to a simple circuit arrangement for limiting the current at make for transformers which only makes the full operating voltage available after a function check of the transformer.
2. Description of the Prior Art
For a brief moment after turn-on, a transformer loads a current source only with the ohmic part of the wire resistor of the transformer's primary winding. The current peak at make is all the higher the lower the impedance of the primary winding. Further, this current peak is all the wider, i.e. all the higher in energy, the larger the iron core to be magnetized. As a result, the main voltage can collapse or fuses can respond. The current at make must, therefore, be limited.
Methods are known to arrange the ohmic impedances between the input of the primary winding of the transformer and the current source so that the current at make is reduced. After turn-on, these resistors lead to the undesired result that an additional voltage drop-off occurs at them. Moreover, the resistors become heated during operation and can be destroyed due to the development of such heat.
Over and above this, it is known to bridge the resistors after a permanently predetermined time, after which one assumes an adequate pre-magnetization of the transformer. For example, the bridging occurs with a time-controlled relay. Due to the fixed time prescription, however, a function check of the transformer is lacking. As such, a main voltage collapse can still occur after the permanently predetermined time in case of a short in the transformer.
Accordingly, an object of the present invention is to specify a simple circuit arrangement for limiting the current at make for transformers that only makes the full operating voltage available after a function check of the transformer.